Electroactive polymers have attracted a great deal of attention due to their promising applications as functional materials for conductive materials, light-emitting diodes, electrochromic devices, field effect transistors, photovoltaic devices, batteries, antistats etc. One important class of inherently conductive or electroactive polymers are electrochromic polymers.
Electrochromic devices are well known, e.g., U.S. Pat. Nos. 4,902,108 and 6,178,034, incorporated herein in their entirety by reference. Such devices undergo a change in electromagnetic radiation transmission upon application of an electrical stimulus and have found use in a number of commercial applications. For example, they may be employed in glazings, e.g., energy efficient and privacy windows for architectural or automotive use, automotive rearview mirrors, displays, filters, eyewear, antidazzle and fog penetrating devices, and other applications where variable light transmission is desired.
Electrochromic devices are typically associated with a noticeable change in color. Changes in other optical properties, such as in the degree of clarity and opacity and absorption in the IR, are also characteristics of such devices.
In electrochromic materials, electrochemical oxidation or reduction induces a reversible change in the reflected or transmitted light. Electrochromic properties have proved especially useful in the construction of mirrors, displays, windows etc where the transparency or color of the article is altered by applying or altering an applied electrical voltage. Commercial devices include rear view mirrors that darken at night to prevent glare from headlights, or windows that darken to reduce transmitted sunlight or to provide privacy.
Many electrochromic devices have been produced using inorganic compounds like tungsten trioxide and iridium dioxide, but organic compounds continue to find increasing use as electrochromic materials such as viologens, metallophthalocyanines and conducting polymers. Advantages of organic materials include different electronic spectra, and therefore different colors, the possibility of lower processing costs and enhanced electrochromic contrast and switching speed. In addition, organic materials can more easily be fashioned into flexible devices such as would be used in electronic paper or other such applications.
Electrochromic materials include compounds that change from one color to another with applied voltage as well as compounds that change from transparent or clear to opaque or colored. The change from clear to colored can occur when a material is electrochemically oxidized, anodically coloring, or when the material is electrochemically reduced, cathodically coloring. The reverse reaction, for example back to clear, should occur when the electrical impulse is removed or reversed.
The terms “anodically coloring” and “cathodically coloring” typically refer to materials which are either lightly colored, colorless or nearly colorless in their neutral state, however more than one color may be formed depending on the applied voltage. For example, an anodically coloring polymer may turn from colorless to red upon application of a particular voltage and the same polymer may then turn from red to, for example, green upon increasing the voltage. Such a polymer is considered anodically coloring because it is clear or lightly colored in its neutral state.
U.S. Pat. No. 6,791,738, incorporated herein in its entirety by reference, provides electrochromic polymers and devices. In particular, anodically coloring polymers having a band gap >3 eV in the neutral state and oxidation potential <0.5 vs a saturated calomel electrode, such as poly 3,4-dialkoxypyrroles, are provided.
Being as co-polymers are a subset of polymers, when used herein “polymers” is a term including both co-polymers and homo-polymers.
Witucki, et. al., U.S. Pat. No. 4,697,000 disclose the production of electronically conductive polypyrroles, which may be co-polymers of variously substituted pyrrole repeating units.
U.S. Pat. No. 5,446,577, incorporated herein in its entirety by reference, discloses display devices comprising a transparent outer layer, a first electrode, which is ion-permeable, having a reflective surface facing the transparent layer, an electrochromic material, preferably a conductive polymer such, as polyaniline, located between the reflective surface and the outer transparent layer, an electrolyte in contact with the electrochromic material and a second electrode located behind the first electrode. The display devices are capable of changing reflectance and/or color by the application of an electric potential to the electrodes.
U.S. Pat. No. 5,995,273, incorporated herein in its entirety by reference, discloses an electrochromic display device having an electrochromic conducting polymer layer in contact with a flexible outer layer; a conductive reflective layer disposed between the electrochromic conducting polymer and a substrate layer; and a liquid or solid electrolyte contacting the conductive reflective layer and a counter electrode in the device.
Inherently conductive materials based on organic polymers offer many advantages over metal or other inorganic materials in electrochromic devices. For example, polymers are often more readily processed providing improvements in device construction. Many conductive polymers are handled easily in air and can be molded or processed using conventional techniques well known in conventional plastic and coating applications. Soluble polymers can be applied as a coating or via an ink jet or standard lithography process.
However, there are also potential disadvantages in using conductive polymers. Often, the polymer must remain in contact with an electrode or other surface. As with inks and coatings, adhesion to the surface must be attained and retained. Poor contact with, for example, an electrode, or subsequent delaminating will negatively impact or negate the desired electrochromic behavior. Further, many electrochromic applications place electrochromic polymer in the presence of electrolyte systems which may include aggressive solvents. The same solubility characteristics that allow a polymer to be applied as a coating may also result in a greater degree of polymer dissolution or delamination.
Many anodically coloring polymers face an additional problem in that the intensity or color strength of the colors that are formed upon voltage application or variation is not particularly strong, particularly when compared with the colors produced using cathodically coloring polymers.
Any attempt to improve the performance of electrochromic polymers, for example, the adhesion or color strength of the polymer, must not impact the characteristics of the polymer that make it valuable. For example, fast switching times, color space and sharp differentiation between the electrically oxidized or reduced forms of electrochromic materials can not suffer as a consequence of improved electrode adhesion.
It has been found that blending electrochromic, anodically coloring polymers with non-electrically conducting binder polymers not only improves film forming properties and durability, but also enhances both the color space available to the polymer and the intensity of the colors formed upon the variation of applied voltage, particularly after repeated switching, without adversely effecting the switching speeds (time to complete the color change) of the polymers. In certain cases, the color space and/or intensity available from the inventive blend, even upon initial switching, exceeds that available from the electrochromic polymer when used alone.